1. Field of the Invention
The invention is generally related to the area of optical communications. In particular, the invention is related to optical space expanders used in optical devices such as multiplexing/demultiplexing or add/drop devices with optical filters in free space and the method for making the same.
2. The Background of Related Art
The future communication networks demand ever increasing bandwidths and flexibility to different communication protocols. Fiber optic networks are becoming increasingly popular for data transmission due to their high speed and high capacity capabilities. Wavelength division multiplexing (WDM) is an exemplary technology that puts data from different sources together on an optical fiber with each signal carried at the same time on its own separate light wavelength. Using the WDM system, up to 80 (and theoretically more) separate wavelengths or channels of data can be multiplexed into a light stream transmitted on a single optical fiber. To take the benefits and advantages offered by the WDM system, there require many sophisticated optical network elements.
Optical multiplexing/demultiplexing devices are those elements often used in optical systems and networks to combine or multiplex a number of optical signals into a multiplexed signal, or separate or demultiplex a multiplexed signal into separate signals. FIG. 1 shows a conventional CWDM device 100 that can be used for multiplexing or demultiplexing purpose. For demultiplexing operation, a multiplexed signal or a light beam is coupled to an input collimator 102. The light beam includes, for example, a number of separate signals, each for a channel or at a particular wavelength, for example, wavelengths λ1, λ2, . . . λN. The light beam is coupled by a collimator 102 to a first filter 104 that is configured to pass all channels or wavelengths except for four selected channels or wavelengths λ1, λ2, λ3, and λ4. As a result, signals for other than the selected channels or at wavelengths other than the wavelengths λ1, λ2, λ3, and λ4 transmit through the filter 104 and outputs via a collimator 106.
On the other hand, signals for the selected channels or at the wavelengths λ1, λ2, λ3, and λ4 are reflected to a filter 106. For simplicity, a channel and a wavelength are interchangeably used hereinafter unless explicitly stated. It is assumed that the filter 106 is configured to pass a wavelength λ1 such that the signal at wavelength λ1 transmits through the filter 108 and outputs via a collimator 110 and others are reflected to a next filter 112. In the similar fashion, each of the filters 112, 114 and 116 are configured respectively to pass a wavelength λ2, λ3, or λ4 and eventually signals at the wavelengths λ2, λ3, and λ4 are respectively output from collimators 118, 120 and 122.
In general, the individual components, such as collimators 102, 106, 110, 118, 120, and 122 as well as the filters 104, 108, 112, 114 and 116, are assembled to a common substrate. The resultant device shall be small enough. When two components are positioned in close proximity, a certain amount of cross talk between the optical paths leading to the components may happen. To minimize the cross talk between two or more optical paths, a light beam is usually impinged upon a filter at an angle that is referred to herein as angle of incidence (AOI). In theory, a large AOI may minimize the cross talk. However, a large AOI would cause the filter to perform undesirably, namely the frequency response of an optical filter depends somehow on the AOI, resulting in unwanted residues (i.e., errors) in filtered or reflected signals. When a number of devices, such as device 100, are cascaded, the errors are accumulated or amplified.
In assembling the individual components to a common substrate, various types of mounting means may be used. One of them is to use wedges to position each of the components to the common substrate. The mounting space sets a limit on a minimum lateral distance adjacent to a component or between two components, thus a minimum achievable beam incidence angle to a filter.
There is another issue related to a loss profile control in the prior art multiplexing/demultiplexing devices. Typically, an optical path length from an input collimator to a receiving channel collimator is very limited, as a result, the collimator characteristics dominates the loss of each wavelength channel. As the collimators are close to filters, no enough distance adjustment space is available. To obtain a desired loss profile, for example, high channel uniformity, one often tries a combination of different types of collimators. However, the use of mixed collimator types may increase the manufacturing and assembling complexity.
Further, another issue associated with the prior art devices is the adjustment of the filter central wavelength. If a filter central wavelength is out of a desired range, the filter may be rotated or adjusted to change the central wavelength. However, after the filter is rotated or repositioned, the output beam from the filter departs from the original direction, resulting in difficulty in manufacturing or assembling. Worse is, as the beam propagates, that the output beam, thus the aligned output collimator is out of the desired position, which can cause significant packaging problems.
Accordingly, there is a great need for new designs that provide small AOL and flexibility in adjustment in devices that are amenable to small footprint, broad operating wavelength range, enhanced impact performance, lower cost, and easier manufacturing process.